Recovery of metals



May 19, 1931.4 ROBINSON 1,805,310

RECOVERY oF METALS l Filed Feb. 16, 192e 7//7 re (ance/)fra fes Fe, Pb, Bf; W, w, f/ 02,61%)

Ess 'rom ROBINSON ATTORNEYS I Patented May 19, 1931 UNITED, STATES PATENT oFFlcE PRESTON ROBINSON, OF NEW YORK, N. Y., ASSIGNOR T GUGGENHEIM BROTHERS, OF

N'EW YORK, N. Y., A COIARTNERSHIP RECOVERY OF METALS Application med February 16, 192s. serial No. 254,888.

This invention relates to the treatment of: tm ores, concentrates, and other tin-'bearing materials, and has for its object the' pro- Vision of an improved method of treating tin-bearing materials for the` production 'of' metallic tin of relatively high purity as.

Well als the recovery in marketable form of the other valuable metals in the tin-bearing material.

Complex tin ores, and in particular lode ores of the cassiterite type, andtheir concentrates, frequently, and indeed usually, contain iron (Fe), tungsten (lV), bismuth (Bi), antimony (Sb), lead Pb), copper (Cu), zinc (Zn), silver. (Ag aluminum (Al), arsenic (As), barium (Ba), phosphorus (P), sulfur (S), silicon (Si), and the like. The iron occurs for the most part as pyrite, wolramite and in complex silicates. It is a very common and troublesome contaminant in tin ores. The tungsten vprobably occurs as the .mineral Wolframite. The remaining impurities are present more or less as oxides and native sultides.v In addition, such complex tin o'res are further complicated by the presence of certain complex silicates, such as biotite and tourmaline. These complex silicatesV are generally ,composed of various combinations Aof silicon with other elements, such, for example, as iron, manganese, titanium, aluminum, and tungsten.

In some of the present customary methods of tin smelting, the tin ore, or its tin-bearing concentrate, is, without preparatory metal-` lurgical treatment, subjected to the smelting operation. In this smelting operation, a considerable part of the tin is lost in the slag, and a furtherconsiderable part of the tin goes into a metallic alloy (with iron, tungsten etc.) known in the industry as hardhead, while such metallic tin as is produced by the operation isrelatively impure and must be subjected to a subsequent reining treatment, such for example as electrolysis, poling, etc. The iron in the smelting charge goes into the slag where it exercises the deleterious influence of drawing or attracting tin from the charge, thereby not only causing an objectionable loss of tin but :forming complex iron-tin silicates which 'have a particularly corrosive action on the tin compounds. A further loss of tin occurs in the flue dust resulting from the smelting operation, so that the ultimate yield of metallic tin, eveneof a relatively impure grade, is considerably less than should be expected in commercial practice. In addition to these difficulties, the present practices of tin smelting require excessively long periods of time, and multi-stage Vsmelting operations are generally necessary to avoid undue losses of tin. Y f

The present invention in its complete aspeet contemplates an improved method of recovering tin from tin-bearing materials involving the treatment of the material in two stages, the first of. Wliich is a preparatory purification treatment and the second paratory purication of the tin-bearing ma- .terial is preferably conducted so as torp'roduce a purified tin product capable of forming a tin-smelting charge having a relatively high sintering temperature from which substantially pure metallic tin can be obtained by direct smelting with the production of a substantially non-ferruginous neutral slag of good fluidity.

Thus, in accordance with the invention, the raw tin-bearing material is first subjected to a preparatory purification treatment in the course of which the objectionable impurities (with respect to tin-smelting) are in very large part eliminated and, where valuable, substantially recovered in marketable forms. In this preparator purification treatment, the complex silicates and tungstate's are so broken up or modified that the resultant purified material has a relatively high` sintering temperature in the subsequent smelting operation. Moreover, the purified tin-bearing material is of such character that when subjected with appropriate fluxing agents to a smelting operation, a substantially non-ferruginous slag is hy rochloric acid, andthe tungsten nmayv then be precipitated with ferrous-chloride..

to form ferrous tungstatea cedure itselfand perhaps to a smaller extent by the removal of the impurities'. The

obtained,thereby avoiding loss of tin in the.-

slag and undue corrosion of the furnace lining. In Athis smelting operation a very small amount of .h's'irdheadv is formed and very little tin is lost in thesl'agl and a high recovery of metallic tin of high purityis ric, nitric, hydrochloric, or the like, or mixtures of such acids, at an elevated temperature, referably in excess of 100 C., for

.ya suiiclent length of time to convert the objectionable impurities into compounds that may be separated from the tin-bearing material' by leaching. Following this acid treatment at elevated. temperature, the tinbearing material is cooled and successively leached, referably at atmospheric tempera-- ture, with salt and alkali solution such, for

example, as sodium chloride and sodiumhydroxide solutions, to extract compounds of .as silver-chloride (AgCl), bismuth oxy lead, iron, bismuth, copper, arsenic, silver, tungsten, antimony and the like. 'Ifhe re` sultlng purified tin-bearing materlal isthen dried and subjected tovappropriate treat-J ment for the recovery of -its tin content, as

by smelting or gaseous reduction.-

The leach liquors obtained ,from 'leaching the acid-treated -tin-bearing material with sodium chloride are appropriately treated to recover the valuable metals contained therein. Thus,-the leached liquors may be diluted with water to precipitate certain of the metals, such as silver, bismuth and lead,

chloride (BiOCl), and lead sulfate (PbSOi), and a small amount of tin which may have beenpresent in soluble form in tlie raw tinbearing material or in the hardhead or'flue dust subjected to the' acid treatment. This f metal-containing precipitate may be appro- (Feen priately treated for the recovery ofits valuable metal contents.

' The sodium hydroxide leachliquor may be treatedby known methods to recover tungsten therefrom. For example, vthe liquor, whichv contains an excessof alkali,

ma be neutralized with an acid, such as The tin-bearing material should be finely ground or crushed prior to theV preparatory purification .-treatment. Tin, concentratesl are by reason of their relatively inestate vof subdivision favorably -condition'ed for this treatment.

Hardhead, when subjected to the preparatory purification treatment -of the invention, is first roasted at a temperature of approximately 50G-800 C., in the presence of airfor a suiiicient'length of time to render the resultant tin oxide insoluble in the leaching liquors after the sulfuric acid treatment.' At the `same time, the 4iron in the hardhead'is oxidized to the ferrie state, but

since too high a temperature and too long a time of treatment decreases the solubility of the iron, care should. be taken to keep the oxidized iron in a soluble condition so that it will be removed in the subsequent leaching step. The tungsten present in theI to condition both -hardhead and flue dust for the preparatory purification treatment of the invention.

The' urified tin-bearing material is now ready orA the tin-recovery operation. In accordance with my preferred practice, the material is' mixed, without drying, with luxing agents of appropriate composition and amount and with carbonaceous reducing lagent and briquetted, with the aid, if necessary, of an appropriate binder, such -as molasses, sulite waste liquor, or the like.

The briquettes are then appropriately dried .and smelted Yin an electric furnace of the slag-resistance type. The smelting temperature is around 12009 C., andthe character of the tin-bearing material Vandthe amount and composition of the iuxing agents are such that a non-ferruginous slag of god iluidityis formed at a temperature slightlyl in excess of 1100 C. In this smelting operation a relatively smallamou-nt of hardhead, is formed, While a very satisfactory recovery ofI metallic tin of high purity is ob!- tained.

The preparatory-purilication treatment of the'mvention materially raises the .s intering A temperature jof the purified tin-bearing material with `respect .to the raw` tinbearing material. This raising of the'sintering point of the tin-bearing material is believed to be largely eii'ected by the purification proactionseemsto be largely due to the breaking. material. As a consequence'of the Araising of the sintering ltemperature of the tinfavors economic utilization of the reducing agent, since maximum converson of carbon monoxide to carbon dioxide is thereby obtained compatabile with equilibrium conditions, and, in the case of electric smelting, lower power consumption becomes possible.

Tin smelting operations are best carried on with slags of relatively high fluidity, be-

cause the metal to be recovered separates more readily from such a slag. A mobile slag, moreover, is easily removed from the reduction chamber and does not tend to clog the reduction apparatus. The presence of iron in the slag tends to give the slag high fluidity. However, fluidity obtained in that manneris objectionable for the reason that the iron attracts considerable tin to the slag, with an attendant high loss vof tin. Under ordinary conditions of tin smelting, if the viron is removed from the tin-bearing -materialprior to smelting, the slag will be viscous and pasty.

In accordance with the preparatory purification treatment of the present-invention, substantially all of the iron compounds are removed from the tin-bearing material before the smelting operation. Hence,`under ordinary tin-smelting conditions, the resulting slag would be viscous and pasty. However', the present invention contemplates the provision of certain substitutes for the iron which will give the slag' high fluidity and which will, moreover, exert certain other desirable features on the smelting operation, without having any ofthe bad features of the iron. -The slag will be neutral. Its weight will be materially less than that heretofore obtained, and its composition will be such as to greatly speed up the action of reduction. v

Fluxing materials, such as`alkalies and 'alkaline-earths, added to the tin smelting charge in appropriate proportions, the resultant slag in the smelting operation high fluidity, inthe absence of appreciable amounts of iron compounds. This condition may be obtained by the addition of proper relative amounts of. Na2O (sodium oxide) and CaO (calcium oxide) preferably in the form of soda ash and lime, to the charge. The relative amounts of soda ash and lime to be added to the smelting charge are de endent upon, land influenced by, various factors, such as the complex nature of the various ingredients present' in the will give charge, the temperatures to be employed,

etc; In order todetermine the approximate amounts and relative proportions of Na20 and CaOfor the purpose, recourse may be had to curves obtained when plotting fusion temperatures of various mixtures of sodiu/m silicate (Na2SiO3) and calcium silicate (CaSiOs). Two eutectic points are determined in this manner, one at about 1120C. (with about 27 mol. of NazSiOs and about 73 mol. of CaSiOa) and the other at about 930 C. (with about 8,0.y mol. of Na2SiO3 and about 20 mol. of CavSiOs). Since the fusion temperatures for amounts of N azSiOs slightly less or greater than in the first eutectic (1120 C.) are not much larger than 1120 C., a slight deiciency or excess of Na20 may be employed Without making a much higher temperature of fusion necessary` Since the temperature necessary to obtain the second eutectic,

(l, 120 C.) is to be preferred in the tin smelting operation because it insures a more rapid reduction of the tin compounds. factors being equal, the higher the temperature the more rapid the rate of reduction. Moreover, vwith the use of a temperature around ll20 C., a minimum amount of N a2O may be employed to obtain a liuid slag. This is economically desirable for the reason that NazO, usually in the form of soda ash, is comparativel expensive. The presence of a2O and CaO in proper proportions to the silica in the tin smelting charge not only gives the resulting slag high fluidity, but also tends to make the slag neutral or non-reactive. Sulicient amounts are added to make the slag neutral. This is desirablev for the reason that a neutral slag has less tendency to hold tin as compounds than does an acid or basic slag. If the slag is basic, stannates will form and they exert a" corrosive action on the furnace lining. In thevcase of an acid `slag, tin silicates will form and these also corrode the lining.

On the whole, it is preferable-to obtain neutral slags and employ a neutral furnace lining. If a'basic or acid lining is employed, it will react with tin in the charge. to form stannates or tin silicates just as 1n the case offbasic and acid slags, which also exert a corrosive action on the rest of the lining.

The improvements of the present invention alsoenable the tin smelting operation to be conducted with the formation of a smaller amount of slag than heretofore obtained. This is desirable for the reason that since the smelting operation may be conducted with the formation of less slag there is a correspondingly less opportunity for loss of tin in the slag. The loss of tin in slag is a function of the weightof the slag. A sav-l ing of tin is thereby effected. It is of course obvious that with less slag, the metallurgical operation may be conducted more efliciently and rapidly f Other An important factor in reduction operations is the speed at which the reaction takes place. In the practice of the present invention the purified tin-bearing material can be reduced in about 1` hour, 'whereas it requires 3 to 6 hours to effect the reduction in the same type of furnace of the unpurified tin-bearing material with fluxes of Athe kind generally employed, such as lime or lime andluorspar. I believe this accelerathe catalytic action of the Nago present in the charge as a fluxing agent. 'At temperatures approaching reduction of the tin compounds, the Na2O is reduced by the carbonaceous reducing agent to form metallic sodium This free sodium in turn is very reactive and promptly combines yvith the tin oxide in the charge to 'form metallic tin With .these silicates broken up or modified,

the carbon monoxide gas 1n thereduction chamber. has relatively easy access to the I tin compounds to reduce the same to metal- -lic tin Electric furnaces of the. slag bath type or of the high frequency induction type haveproved particularly adaptable for re- 4` ducingv or smelting the purified tin-bearing, o

material. O n the other hand, any type of reduction furnace in which sullicient heat can be supplied to effect the reducing operation Will be found adequate for the smelt- 'ing or reduction of the purified tin-bearing material. While the reduction or smelling 0 eration may be conducted Yon batches of c arge material, rials in the manner indicated makes the practice of the invention peculiarly adapted I to continuous operation.v Molten slag and metal 'may be separately withdrawn as fresh chargematerial is introduced into the furnace. A slag of `very low metal content and al minimum amount of hardhead will result. The slag will be substantially nonferruginous. 'A maximum separation of tin from hardhead may be obtained by ccntrifuging the molten metal mixture at a temperature slightly above the melting point oftin. y

The raising offthe sintering point of the tin-bearing material is particularly advantageous in gaseous reduction processes` because the tin compounds present may be -reduced to molten-tin metal before decreas- -companying the. use of purified mateing the active surface material by sintering. tering also countered in gaseous reduction processes, such as clogging of the reduction appara.-

of the tin-bearing Prevention of. sin- `tus. The practice of the invention 1s par- Vdescription of the treatment of a representative .charge of tin-bearing material in aclcordance with the principles of the invention. Numerals are used to designate the various main steps or operations of the process. Each succeeding paragraph .describing the various steps or-operations is numobviates other difliculties" enbered to correspond vvith numbers in the boxes on the accompanying {iowsheet which indicate similar steps or operations;

It Will ofcourse be understood following detailed. description andthe acflowsheet are provided for-purposes of illustration only, and they are not` .restrictive of the' invention.

(1) 148 lbs. of a blend of unpuriied Bolivian tin ore concentrates, assaying as follovvszth/at the Per cent Sn 60. 0 Fe- 4. 55 S- 0. 845 y, Pb 0.216 B i j 0; 326 W 1.69 Cu y 0. 031l SiO2 4; 77 As Y 0. 383' Sb 0.037 and crushed so that 85% passed throughva 200 mesh screen, were agitated with 272 lbs. of 66 B. sulfuricacid for 6 hours at 200 C. in an iron kettle. The mixturewas ltered lhot on a porous silica plate filter, removing 139 lbs. excess'sulfuric acid. The cooled ilter'cake wasthen agitated lvigorously for 2.5 hours in a wooden tankV with 264 lbs. of a 20% sodium chlorideA solution, filtered and washed on the iilter with 77 lbs.of water, again agitated for 1.25 hours with 132 lbs. of 20% sodium chloride solution, filtered, and againagitated 'f0`r11.25 hours with 132I lbs. ofV 20% sodium chloride solution containing 5.2 lbs. of 20 B. commercial yhydrochloric acid, ltered and Washed three times on the filter with 77 lbs. of Water.

(2) The combined vsodium chloride leach solutions and Washes (from 1 above) are' treatedforthe recovery of dissolved values by known methods. In this particular run all ofthe' bismuth was precipitated by bringing the solution into contact successively with scrap iron and iron sulfide.v In .other runs, the bismuth was recovered as bismuth oxychloride by diluting the solution with water.

(3) The partially vpurified concentrate (from 1 above) containing about 8% mois- .iso

ture was agitated for 2.25 hours with 132 lbs. of 4.2% sodium hydroxide solution, filtered and washed 3 times with 47 lbs. of water. This completed the the tin-bearin material.

. (4) The so ium hydroxide leach solution (from 3 above) assayedas follows:

Grams per liter Sn 0.02 W 13. 05 Si02 1.19 Pb 0.002' NaOH 35. 5

Thissolution and the immediately follow# ing washeswere treated for the precipitation of tungsten as calcium .tu-ngstate by neutralizing with hydrochloric acid and adding calcium chloride in accordance with well known practice.

(5) The purified concentrate from .3 above was dried and assayed. The results of this assay and the composition of the raw concentrate are given below for comparison:

Raw Purified concen conoen Etgc' trate trate Per cent Per cent 68. 08 1. 22 76. 0. 67 30. 0. 035 86. 0.003 99. 0. 08 96. 0. 005 98. 4. 82 8. 5 0. 058- 87. 0. 000 100.

The purifiedconcentrate was mixed with 0.17 lbs. of minus 14 mesh coal per pound `of tin in the concentrate; and 0.7 lbs'. of

lime and 0.5 lbs. of soda ash per pound of silica/in the charge for fluxes. To the abovemixture 4% by weight on the -concentrate of a coal tar pitch'was added and the whole Q usual manner.

charge thoroughly mixed and madeinto briquettes. The quantities of fiuxes indicated are. so proportioned as to form an eutectic of Na2SiO3 and CaSiOa.- This combination of NazSiOa and CaSiO3 has been found to give a Huid Slag even when as much as 20% of alumina (A1203) is present in the slag. The briquettes were then charged into an electric smelting furnace of the slag resistance type, smelted continuously and theslag and metal tapped in the The time for reduction was about 1 hour. The heretofore customary purification of practice of reducing the unpurified concentrates With the usual fiuxes in a reverberatory furnace requires a very considerably longer time.

(6) The molten slag (from 5 above) was run into a settling pot, and after standing for 4 hours at approximately 12.50 C. the slag was drawn off. It assayed 0.5% tin. This slag is substantially non-ferruginous. The slag obtained in the more customary practices of smelting unpuriicd tin concentrates in a reverberatory furnace contains from 15-25% iron,`while the slag obtained in the practice of the present invention contains but 2 to 4% iron-depending on the amount of iron present in the purified tin concentrate.

(7 )l The molten metal (from 5 above) `was allowed to/cool to above 30000., and

wasl then centrifuged in an iron centrifugal heated just above the melting point of tin. This centrifuging operation quite effectively separated the substantially pure molten tin metal from the irontungsten-tin.alloy, generally referred to as hardhead.

(8) The solid alloy metal or hardhead (from 7 above) was then roasted in air for twoto four hours at 600800C. in order to convert the metals into oxides and to render the tin oxide formed insoluble, but at Per cent Bi 1 0. 005 Pb 0. 052 Cu 0. 007 As 0. O85 Sb., 0. 0001 Sn (by difference); 99. 851

To further illustrate the advantages of purifying the -tin concentrates in prepara- @on for the smelting operation, the following example is given: The unpurified tin concentrate assaying 11.5% iron was treated with 380 lbs. of "4 B. sulfuric acid for each 1000 lbs. of tin concentrate. fThe so- -treated material was leachedas hereinbefore described, and a purified concentrate assaying 1.33% Fe was obtained. This purified concentrate was smelted as hereinbefore described, and a final tin product equivalent in purity to that in the aforementioned example resulted. If, on'the other hand, ,a

' charge of this same'tin concentrate, in its unpurified state, is directly subjected to a smelting operation, only slag and hardhead are formed, and all of the available tin j finds its Way into the slag and hardhea'd.

The process of the invention is adaptable to .tin concentrates regardless of previous treatment. Thus, with a roasted, Weathered or oxidized concentrate satisfactory results are obtained by treatment with 60 (B.

sulfric acid at 150 C., while unroasted,

added tothe fluid pulp when using a large excess of sulfuric acid to increase the solubility of the barium sulfate. This barium sulfate in solution'may subsequently be precipitated from the sulfuric acid filtrate by adding water in the form of more dilute f sulfuric acid, until a composition is reached bearingmaterialcontaining tungsten com` at which barium soluble.

" I claim: l 1. The method of recoveringtin from tinpounds which comprises eliminating substantially all of the tungsten from the material'yby treating the material successivelywith acid and alkaline solutions, and subjecting-the material thus substantially freed of tungsten to a reducing operation in which v a higher recovery of tin is directly effected asa consequence of the tungsten elimination.,

2. The method of recovering tin from tin- Vmg the-material for smelting by a preparatory purification treatment, and smelting the so purified tin-bearing material in the pres-l land alkaline-earth fluxing-agents to yield a substantially non-ferruginous slag of high fiuidity and thereby obtaining a relatively high yield of tin metal.

3. The method of recovering tin from tin- 4loearing material which comprises conditioning the material for smelting by a preparatory purification treatment, and smelting the so purified. tin-bearing material in the presence of a carbonaceous reducing agent and appropriate relative proportions of alkali and alkaline-earth iluxing agents to yield a substantially neutral slag of high sulfate is relativelyl ifnbearing material which comprises conditionfluidit and thereby obtaining a` relatively hig yieldof tin metal. y

4. bearing material which comprises conditioning the material for smelting by a preparatory purification treatment, and smeltlng the so purified tin-bearing material in the presence of a carbonaceous reducing agent and appropriate relative proportions of alkali and alkaline-earth-iiuxing agents to yield a slag of high iiuidit with `a minimum amount of the alkali uxing agent. l

5. The method of recovering tin from tinbea'ing material which comprises conditioning .thefmaterial for smelting by a preparatory purification treatment, briquetting the so purified tin-bearing material admixed The method of recovering tin from tini with carbonaceous reducing agent andalkali and alkaline-earth fluxing agents, andl subiuxing agent, and subjecting said briquetted mixture to a sufficiently high temperature to reduce compounds of tin'and yield a slag l of good iuidity.

7. The method' 'ofA recovering tin from tin-bearing material which comprises conditioningl the material for smelting by a preparatory purification treatment, briquettino the so purified tin-bearing material admiixned with carbonaceous reducing agent and alkali and alkaline-earth iiuxing agents, with the alkaline-earth fluxing agent present in preponderating amount with respect to the alkali luxing agent, and subjecting said briquetted mixture to a sufiiciently high temperature` to reduce compounds of tin' and yield a slag of good iiuidity. Y

8. The method of recovering tin` from tin-bearing` material which comprises converting substantially. all of the objection. abld impuritiesin the tin-bearing material into soluble compounds and breaking up complex silicates present therein by subjecting wthe material ina finely divided state and at an elevated temperature to the action `of concentrated sulfuric acid, removing said soluble compounds from the tin-bearing material by a leaching operation, and smelting the so purified tin-bearingvmaterial in the presence of appropriate amounts and compositions of reducing and luxing agents to yield a high recovery vof tin metal and a slag of good'iiuidity. i

.9. The method of recovering -tin from tin-bearing material which comprises subjecting the material to a two-stage treatment in the first stage ofwhich they matcrial is conditioned for reduction by preparatory purification involving treatment of the material with acid at an elevated tempel-ature 'followed' by leaching of the so treated material with a saline solution whereby a substantial elimination of iron is effected and in the second stage of which the so purified tin-bearing material is subjected io a reducing operation.

10. The method of recovering tin from tin-bearing material which comprises subjecting the material to a two-stage treatment in the first-stage of which the material is conditioned for reduction by preparatory purification involving treatment of the material with acid at an elevated temperature followed by successive leaching with a saline solution and an alkaline solution whereby substantial elimination of iron and tungsten is effected and in the second stage of which the so purified tin-'bearing material is subjected to a reducing operation.

11. The method of recovering tin from tin-bearing material which comprises subjecting the material to a two-stage treatment in the first stage of which the material is conditioned for reduction by preparatory purification `involving treatment of the ma, terial with sulfuric acid at an elevated temperature followed by successive leachings with a saline solution and an. alkaline solution whereby the sintering temperature of the material is raised and substantial elimination of iron and tungsten iseffected and in the second stage of which the so purified tin-bearing material is subjected to a reducing operation.

l2. The method of recovering fin Vfrom tin-bearing material which comprises subjecting the material to a two-stage treatment in the first stage of which the material is subjected to a preparatory purification treatment Wherebya substantial elimination of iron is eii'ected and in the second stage of which the so purified tin-bearing material is subjected to a smelting operation with a combination of alkali and alkalineearth fluxing agents capable of producing a fiuidslag at the prevailing temperatureof tin reduction.

13. The method of recovering tin from tin-bearing material which comprises subjecting the material to a two-stage treatment in the first stage of which the material is subjected to a preparatory purification treatment whereby a substantial elimination of iron is effected and' in the second stage of which theA so purified tin-bearing material is subjected to a smelting operation with a combination yof alkali and alkaline-earth fiuxing agents of which the amount'l of the f alkaline-earth agent. employed is largely in excess of the amount of alkali agent employed.

14. The method of recovering tin from tin-bearing material which comprises subjecting the material to a two-stage treatment in the first stage of which the material is subjected to a preparatory purification treatment whereby a substantial elimination ot' iron is effected and in the second stage of which the so purified tin-bearing material is subjectedy to a smelting operation with a combination of alkali and alkalineearth fluxing agents of which the relative amounts employed (with respect to the alkali oxide and alkaline-earth oxide) are approximately in the ratio of one to four respectively. Y

.15. The method of recovering tin from tin-bearing material which comprises converting substantially all of the objectionable impurities in the tin-bearing material into soluble compounds and breaking up complex sllicates present therein by subjecting the material in a finely divided state and at an elevated temperature to the action of an acid, removing said soluble compounds priate amounts and compositions of reduc-v ing and fiuxing agents to yield a high recovery of tin metal and a slag of good fluidity.

16. The method of recovering tin from tin-bearing material which comprises subjecting the material to the action of a concentrated acid at an elevated temperature to convert insoluble iron compounds contained therein to a soluble form, leaching the thus treated material to remove the soluble iron compounds, and smelting the resulting material in the presence of a carbonaceous reducing agent and appropriate amounts of alkali and alkaline-earth fluxing agents to produce a luid'slag in the absence of iron. j

17. The method of recovering tin from tin-bearing material which comprises subjecting the material to the actionof concentrated sulphuric acid at an elevated temperature to convert insoluble iron compounds contained therein to a soluble form, leachleaching 'the thus treated material successively with saline and alkaline solutions to remove Athe soluble iron and tungsten compounds, and smelting the resulting material E in the presence Aof a carbonaceous reducing agent and appropriate amounts of alkall andV alkaline-earth uxing agents to produce a Huid slag in the absence if iron.

Y19. The method of recovering tin from ya tin-bearing material which comprises subjecting the material to the actionof concentrated sulphuric acid at an elevated temperature to convert insoluble iron and tungsten compounds contained therein -to solu- 15 ble forms, leachingI the thus treated material successively with saline and alkaline solutions to remove the soluble iron and tungsten compounds, and smelting the resulting material in' the presence oa car- 2c bonaceous reducing agent and appropriate I amounts of alkali and alkaline-earth flux'- ing agents to produce alluid slag- 1n the absence o f iron.

In test1mony whereof I aiiix my signature. 25 PRESTON ROBINSON. 

